On very rare occassions, as you progress through the levels in the game, you will be thrown into a Rumpus Room rather than the next part of the level you're on. In this small room, you will have to face numerous Pokémon from the level within the close confines of the room. There are two types of Rumpus Room that you can enter.
Rumpus Room Type 1
The first type is relatively standard where you'll only encounter the variety of Pokémon found within the level itself. These Pokémon will respawn until you have faced around 50 of them. Defeat them all, and a trampolin will appear which will take you back up to the level.
Rumpus Room Type 2
The second type will not bring a variety of Pokémon but instead choose an evolution line. With this, the highest evolution of the Pokémon appear as four large toys, similar to bosses and the lower evolution appearing and continuing to respawn until you have defeated the four boss Pokémon.
You have the ability to earn the toys of the Pokémon you face in this mode and the money you receive from the Pokémon is quite high in quantity.